Spaced out A weird zebra-stripe pattern discovered in Earth's inner Van Allen radiation belt is generated by the planet's rotation, according to new research.

The study, reported in the journal Nature, changes science's understanding of Earth's radiation belts, and may provide new insights into the complicated dynamics of similar belts around other planets.

It also overturns previously held views that they were caused by geomagnetic storms from the Sun.

"These radiation belts were discovered more than 50 years ago, so it's very rewarding and exciting to find something completely unexpected," says the study's lead author Dr Sasha Ukhorskiy of Johns Hopkins University in Maryland.

"These were some of the first scientific discoveries ever made in space, and we're still finding out new things."

Earth's Van Allen radiation belts are composed of electrons and ions held in place by the planet's magnetic field.

Early data from NASA's twin Van Allen Probe spacecraft detected the unusual zebra-stripe pattern in the energy spectra of electrons in the inner radiation belt.

The pattern is caused by different electron energy densities at different altitudes.

"A cross-section of these stripes shows a recurring regular up and down pattern of electron energy intensity and flux distribution," says Ukhorskiy.

"This happens because the inner belt is very stable and not as dynamically variable as the outer radiation belt, or the recently discovered third belt."

The phenomenon was thought to be generated by interactions with space weather events from the Sun, such as Coronal Mass Ejections, and the strength of the solar wind flux.

Strange discovery

"The pattern was very striking and grabbed our attention, so we started digging into it," says Ukhorskiy.

Ukhorskiy and colleagues noticed the zebra stripes weren't affected by changing space weather conditions, instead remaining stable at altitudes within about 13,000 kilometres, and at low latitudes in relation to Earth's magnetic axis.

"We started noticing that the zebra stripes were constantly present, and appeared more ordered during quiet geomagnetic conditions, which prompted us to look for a different mechanism or energy source," says Ukhorskiy.

According to Ukhorskiy, the only energy source not related to the Sun, which could influence the belts, is the Earth's rotation.

Although planetary rotation is known to play a role in driving radiation belt dynamics around Jupiter and Saturn, it has been thought inconsequential for Earth's radiation belts.

The authors developed a test-particle simulation model to fit the data.

They discovered Earth's rotation was inducing variations in magnetic and electric fields which interact with the inner belt electrons on a 24-hour cycle, reproducing the zebra stripe pattern.

"It's a very low energy level, but the electric field induced by Earth's rotation permeates throughout the entire spatial distribution of these particles," says Ukhorskiy.

"So while it can't push particles very fast, it does cause a resonance to build up as the particles slowly drift around Earth under the influence of the planet's magnetic field."

Jupiter next stop

Ukhorskiy says the next step will be looking at what's happening on the outer planets Jupiter and Saturn where rotational forces are much more prominent in controlling radiation belt particles.

"We have a similar experiment aboard NASA's Juno mission which is on its way to Jupiter," says Ukhorskiy.

"This will look for similar effects in radiation belts caused by planetary rotation."

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